MORPHOLOGY OF ANGIOSPERMS. 



141 



flower matures first, and after it withers the stamens protrude and 

 discharge their pollen. It is evident that the flowers can not be 

 self-pollinated, nor is it likely that one flower will be pollinated 

 by another of the same spike. (3) The stamens and pistils of the 

 same flower may var}- in length, as in Polygonum (Fig. 87, illus. 

 I and 2) and Lythrum (Fig. 87, illus. 5), or stand in such other 

 relation to each other that self-pollination will not be effected, 

 as in some of the irregular or zygomorphic flowers, like those of 

 Orchids. In these several cases the pollen grains either fall upon 



Fig. 86. Cross-pollination through the agency of a bee, in flower of quince {Cydonia 

 vulgaris). A, flowering branch; B, flower showing bee extracting nectar, and masses of 

 pollen adhering to its legs, some of which will fall upon the stigmas of other flowers when 

 it visits them; C, ripe inferior fleshy fruit (pome) of quince, — After Dodel-Port. 



or are carried b}' various agents to the stigmas of other flowers, 

 and this is known as cross-pollination, and the fertiHzation 

 which follows as cross-fertilization. 



Cross-fertilization is an advantage to the species for usually 

 the seeds which result from this process give rise to plants which 

 are more vigorous and otherwise superior to those which result 

 from self-fertilization. In some cases in order to insure the pro- 



